This invention relates to slide coating apparatus and in particular to a slide coating apparatus wherein the slide surface angle and takeoff angle lie within predetermined limits.
A conventional multi-slide coating apparatus, as disclosed in U.S. Pat. No. 2,761,417 (Russell et al.), performs coating operations by metering individual layers of a coating fluid from a supply, through a trough disposed inside the hopper and then through a narrow distribution slot. A coating fluid is distributed by the slot as a layer uniformly across a downwardly inclined slide surface. Such layer of coating fluid flows by gravity down the slide surface and meets with the adjacent underlayers of coating fluid which have been similarly metered and distributed through narrow slots. The combined coating fluid layers then flow down the slide surface and form a coating bead. The web to be coated is carried by a backup roll and is moved across the bead. The fluid layers in the bead impinge upon the moving web which picks up the multilayer coating fluids from the slide surface.
A cascade coater is a slide coating apparatus in which the impingement point between the multilayer coating fluids and the web is located above the horizontal center line of the backup roll supporting the web to be coated. Exemplary of such a device is that shown in U.S. Pat. No. 3,289,632 (Barstow). British Patent Specification 1,268,144 discusses the desirability of having the slide surface of a cascade coater define an angle of less than 30.degree. with respect to a predetermined horizontal datum.
It has been observed that defects in coating quality are related to the magnitude of a takeoff angle defined between the slide surface and a tangent erected at the point of impingement of the coating fluid onto the backup roll. In an effort to overcome such defects U.S. Pat. No. 4,283,443 (Choinski) discloses a cascade coater having a slide surface with a lip disposed at the lowermost end thereof which lip changes the direction of flow of the fluid layers to a generally upwardly direction aligned with the upward direction of the web movement.
In a slide coating apparatus one or more fluids to be coated on a moving web are accelerated toward the moving web by gravity. Due to the inclination of the slide surface with respect to a horizontal datum the gravitational force imposed on the coating fluid is resolvable into two components, one parallel to the slide surface and the second perpendicular thereto. The momentum of the coating fluid as it impinges upon the moving web is related to the component of the gravitational force parallel to the slide surface. It is desirable that the momentum of the coating fluid be as high as possible in order to overcome the forces which oppose the deposition of the coating fluid onto the web. These opposition forces include forces due to the surface tension of the coating fluid, high shear forces due to the velocity of the web, and forces due to air drawn along the surface of the web under the bead of coating fluid.
The perpendicular component of the gravitational force acting on the coating fluid as it moves down the slide surface tends to even the distribution of the fluid across the transverse dimension of the slide, an effect known in the art as "leveling". In addition, the component of force perpendicular to the slide surface tends to maintain the transverse dimension of the coating fluid and prevents a shrinkage or "neck in" of the transverse dimension of the coating fluid at a given point. In order to maximize the leveling effect it is desirable that the component of the accelerating force perpendicular to the slide surface be as large as possible. However, increasing the perpendicular component has the necessary consequence of decreasing the parallel component thus decreasing the momentum, and vice versa.
Accordingly, it would be desirable to provide a slide surface coating apparatus wherein the component of accelerating force perpendicular to the slide surface and the component parallel thereto are both optimized so that the maximum impingement momentum is achieved while still maintaining an adequate leveling force.